Isopulegol of formula (II) (2-isopropenyl-5-methyl-cyclohexanol) is a terpene that is used as an aroma compound, to generate “flower notes”. Moreover, it is an intermediate in the synthesis of menthol from citral.

Isopulegol isomers occur in nature in a large number of essential oils. As isopulegol is formed relatively easily from citronellal, the compound of formula (I) (3,7-dimethyloct-6-en-1-al), it often occurs accompanying citronellal or is formed during extraction of the essential oil. Isopulegol, which is produced industrially from (+)-citronellal, is as a rule a mixture of different isomers with a high proportion of (−)-isopulegol.
The industrial production of isopulegol is mainly carried out by the chemical cyclization of (+)-citronellal. Originally 80-85% pure raw material obtained from citronella oil was used. Since the 1990s this has increasingly been replaced with the optically purer (+)-citronellal (97.5%) from the so-called Takasago process. Here, geranyldiethyldiamine is isomerized asymmetrically to (+)-citronellal using an Rh-BINAP-complex catalyst (Rh-complex with 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl). The chemical synthesis of isopulegol starting from citronellal has been described many times. (+)-Citronellal can be cyclized using a copper-chromium catalyst, zinc bromide, alkylaluminum chloride, a rhodium complex, a solid acid-base catalyst, zeolite or silica gel. In recent times the silica gel method has increasingly been superseded by the method with zinc bromide, as the latter has higher selectivity.
The cyclization of terpenes with the aid of special cyclases is generally known. For example, in nature squalene is cyclized by a squalene-hopene cyclase (SHC) to the pentacyclic hopene.
The gene and protein sequences of squalene-hopene cyclase derived from the bacterium Zymomonas mobilis (Zm-SHC) are known (Genpept Accession No AAV90172 2004 and Nat Biotechnol 2005, 23:63-68, cf. SEQ ID NO: 1 and 2).
In international application PCT/EP2010/057696 (WO2010139719 A2), to the complete disclosure of which reference is expressly made herein, polypeptides are proposed as biocatalysts for the cyclization of homofarnesol to ambroxan.
The biosynthesis of numerous monoterpenes in the corresponding production organisms has already been elucidated. Frequently this involves cyclization of linear precursor molecules by highly specific biocatalysts. The precursors are generally esters of linear terpene alcohols and diphosphoric acid. One typical example of such a precursor is geranyl pyrophosphate. The pyrophosphate group is eliminated from the molecule enzymatically, and is subsequently hydrolyzed into two phosphate ions.
On the other side, a carbocation is formed, which is then able to undergo further intramolecular reaction and which recombines to form a cyclic monoterpene, with elimination of a proton, for example (Curr. Opin. Chem. Biol. 2009, 13: 180-188).
A problem to be solved by the present invention, furthermore, was to find an alternative to the known chemical cyclization methods for terpenes, allowing terpene compounds to be cyclized by means of enzymatic catalysis, such as the linear citronellal to be cyclized to isopulegol, for example. The problem to be solved by the present invention was furthermore to provide novel biocatalysts that can be used for the cyclization of terpenes, for example of citronellal with formation of isopulegol.